Self-moving displays are often used as toys, decorative conversation pieces or advertising media. Such devices are disclosed in my U.S. Pat. Nos. 6,275,127; 6,853,283; 6,937,125; and U.S. Pat. Publication No. 2005/0102869; all of which are incorporated herein by reference.
These devices can have a sealed outer container having light transmissive walls containing a light transmissive liquid which buoyantly supports an inner body which appears to magically rotate on its own with respect to the outer container, or in what appears to be a solid block of clear glass or plastic. The rotation can be driven by an electric motor hidden within the body. The motor can be powered by a battery or in a longer-term manner by light radiation impacting on photovoltaic cells hidden within the body.
One problem can occur due to atmospheric pressure and humidity differences occurring naturally at various localities throughout the world over time. For example, during a winter storm in Denver, Colo. the pressure and humidity may be far less than during the summer in Rio De Janeiro. Due primarily to manufacturing and safety concerns, the outer container of the device is often made of a relatively non-hermetic material such clear acrylonitrile butadiene styrene (ABS). Therefore, changes in atmospheric pressure and humidity can seep through the walls of the outer container and change the water content and vapor pressure of the inner light transmissive liquid. These changes, coupled with changes in temperature can cause the total liquid volume inside the container to become bigger or smaller than the volume available for the fluid. If the liquid volume is bigger, it can cause an overpressure potentially damaging the display, potentially ruining the magical appearance of the device. These changes can also lead to changes in the buoyancy of the inner body when it contains an amount of gas due to a Cartesian diver effect.
Another potential problem is that these devices can rely on an internal compass magnet aligned with an ambient magnetic field such as the earth's magnetic field to act as a source of counter-torque for their internal motors. In such devices there has been a possibility of a magnetic interaction between the compass magnet and field magnets that are used to generate a relative torque as they interact with coils of wire carrying currents of the on-board electric motor. FIG. 1 shows graphically how this magnetic interaction can cause a speed variation in the motor. It is clear that as the driving current to the motor is reduced, the motor will stop at a much lower drive current than it would without this interaction negatively impacting operation in low-light conditions.
This problem can be reduced by designing the field magnet structure to minimize the magnetic interaction and also by mounting the compass magnet far from the field magnets. However, practically speaking, this reduction can be expensive and can detrimentally increase the size of the powering mechanism in a device having limited space.
Therefore there is a need for a self-rotating device which addresses some or all of the above identified inadequacies.